Jessica Alba is opening up about she achieved her post-baby body ... and it sounds kinda painful.

The 31-year-old mother of two daughters, Harper and Haven, talked to Net-A-Porter magazine about her toned, post-birth body. After being asked how she gained her body back so quickly, the actress shared her secret: a girdle.

"It was brutal; it's not for everyone," she told the magazine. "I wore a double corset day and night for three months. It was sweaty, but worth it."

The actress, whose toned figure is touted by tabloids as one of the best in Hollywood, married her husband Cash Warren in May 2008 in Los Angeles. She gave birth to their youngest child, daughter Haven, in August 2011. Big sister Honor was born in June 2008.

The actress, who is known for her work in films such as "Fantasic Four," "Valentine's Day" and "Machete," also talked to Net-A-Porter about her upcoming project -- which she seems to love.

"I just finished the best movie I've ever done," Alba said of her upcoming role in "Sin City: A Dame To Kill For." "It was so opposite to what I do [at home]. My approach to acting now is so different -- I'm fearless because I have nothing to lose. Before [children] my identity was wrapped up in every decision -- now, I don't care. It could be a pile of [expletive] and you could think it was the worst thing you ever saw, but I still had the best experience."

Alba will be reprising her role of Nancy Callahan from the first film. Callahan is an exotic dancer working her way through law school. She was saved by Detective John Hartigan (Bruce Willis) when she was 11 from being murdered by a child molester. The two met up again when Callahan was an adult and Hartigan sacrificed his life to save her from being raped and murdered.

The film is set for release on Oct. 4, 2013 and stars Joseph Gordon-Levitt, Rosario Dawson and Bruce Willis.